1. Field of the Invention
This invention relates generally to work stations or work holders used in indexing semiconductor lead frames through an automatic wire bonder and, more specifically, to a precisor plate employed in such a work holder or work station adapted to be quickly readjustable to accommodate differently configured lead frames.
2. State of the Art
There are a limited number of companies in the semiconductor industry that manufacture automatic wire bonders used to form wire bonds on semiconductor devices. A leading manufacturer of such wire bonding machines is Kulicke and Soffa Industries, Inc. of Willow Grove, Pa., manufacturer of a work holder designated as model number 2810. These automatic wire bonders typically employ dedicated work holders which are adapted to receive a specifically configured lead frame. The work holders are coupled to or connected to at least three mechanical drives, one for advancing the lead frame, one for positioning the lead frame on the bonding station and one or more drives for clamping the lead frame in the station. Heretofore, changing the work holder required skilled technicians to disassemble and reassemble or partially rebuild the work holder to accommodate each differently configured lead frame and ensure that the new, differently configured lead frames will be precisely aligned with respect to the wire bonder. Such a reconfiguration process typically takes an hour or more with current work holders.
A typical work holder for a Kulicke and Soffa wire bonding machine includes a box-like frame attached to the machine through which lead frames are indexed, the box-like frame forming part of the work station at which lead frames are wire bonded. In addition, a precisor plate is attached to a vertical slider, the slider vertically moving the precisor plate beneath the lead frame. The precisor plate, in conjunction with a heat block, typically rectangular in shape, supports the lead frame in the work holder and keeps the lead frame from moving during the wire bonding operation.
An attempt to provide a modular work station or work holder which is easily removed from and replaced in an automatic wire bonder is disclosed in U.S. Pat. No. 4,765,531 (""531 patent) assigned to Kulicke and Soffa Industries, Inc., the manufacturer of the aforementioned model 2810 wire bonder. While the ""531 patent is generally described with respect to Kulicke and Soffa automatic wire bonder models 1481 and 1482, many of the components and their configurations are similar to the components of the model 2810 wire bonder. In the ""531 patent, the problem of precisely aligning the precisor plate and heat block relative to the rest of the work holder is addressed by immobilizing the entire work station while various pins and bolts align and secure these components together and to the wire bonding machine. While the quick change work station described in the ""531 patent may have been an improvement over the work holder stations of its time, the ""531 patent work holder requires separate alignment of both the precisor plate and the heat block.
The work holder that is provided by Kulicke and Soffa with the 2810 model is as difficult, if not more, to align than the work holder described in the ""531 patent. In addition to not providing a work station that can be quickly realigned, the precisor plate provided with the 2810 model wire bonder has moveable pins that must themselves be properly aligned to precisely engage tooling holes in the lead frame. In addition, the heat block must be separately aligned, and the precisor plate itself requires planarity adjustment both right-to-left and front-to-back.
When a wire bonder as herein described is out of service, production on that machine is lost, resulting in a decrease in manufacturing efficiency. Accordingly, in order to avoid down time for realignment of the work holder, such automatic wire bonders are kept on line as long as possible, wire bonding similarly configured lead frames. In order to accommodate low numbers of production items, where only a few parts must be wire bonded, intermittent batch production, where the configuration of the wire bonder is frequently changed, or long production runs, where realignment of the precisor plate must be periodically performed, it would be desirable to provide a quick change precisor plate that can be simply and quickly adjusted.
Accordingly, a precisor plate and method of aligning the precisor plate to an automated wire bonding work station is herein disclosed. A first embodiment of the precisor plate in accordance with the present invention comprises a plate having fixedly attached alignment pins positioned to engage tooling holes in a lead frame or lead frame strip, whether stamped, etched or otherwise manufactured, and a heat block attachment or mounting portion on the plate for attachment of a heat block thereto. The heat block is held in precise position relative to the precisor plate by a pin which extends from the heat block mounting portion into the heat block and an abutment surface adjacent the heat block mounting portion against which the heat block must abut in order to fit onto the pin. The heat block is secured to the heat block mounting portion by threaded screws which fit through holes provided in the heat block mounting portion and into matingly threaded holes in the heat block.
In another preferred embodiment, at least two pins are provided on the heat block mounting portion and engage an equal number of pin holes in the heat block. The pin holes, which have an internal configuration to substantially match the outer configuration of the pins, must engage with the pins and thus properly align the heat block to the precisor plate.
In yet another preferred embodiment of a precisor plate in accordance with the present invention, the precisor plate and heat block comprise a one-piece component such that the heat block is either permanently attached to the precisor plate or integrally formed as part of the precisor plate. Such a device removes entirely the need to align the heat block relative to the precisor plate, or to separately align the heat block to the work station.
The heat block according to the present invention may be provided with various internal passages, in addition to those used for securing purposes. Such internal passages may include bores sized to receive heating elements and temperature measuring devices, such as thermocouples, and passages to provide a vacuum at the wire bonding surface of the heat block to help secure the lead frame relative to the heat block during the wire bonding process.
In order to align and attach a precisor plate of the present invention to a work station, the heat block is attached to the precisor plate (if separate components) and the precisor plate is attached to a vertical slider of the wire bonding machine by inserting screws through holes provided in the precisor plate and into threaded holes provided in the slider. A lead frame is then indexed into the work station. Before tightening these screws into the threaded holes in the slider, the precisor plate is moved until, upon actuation of the slider, the alignment pins of the precisor plate engage the tooling holes of the lead frame without disturbing the position of the lead frame. When the alignment pins can be engaged into the tooling holes without lifting (i.e., disturbing) the lead frame, the precisor plate is in the correct position and can be securely attached to the slider by tightening the screws into the slider.
To facilitate movement of the precisor plate relative to the work station, one or more holes may be formed in the precisor plate into which grasping members, such as pins or threaded screws, may be inserted, These grasping members may be grasped by a user to move the precisor plate relative to the work station. After proper alignment and upon securement of the precisor plate relative to the work station, the grasping members may be removed.